The C Programming Language is a decent overview of the language. After that using it in a real project is the best way to learn. If you know Linux pretty well and know what syscalls are then you might be interested in reading the Linux source code and writing kernel modules (they're written in C). If none of that sounds familiar to you the overhead is probably too high for going that route to make sense.

In general, the reason to use C is because you are doing something that interfaces with the OS. But the problem with software at this level is that because the abstractions are so low level unless you understand something about the design/architecture of the software the code is really hard to understand.

So I would pick some project that is interesting to you and that you have some familiarity with already and try to implement a simple version of that in C. It is actually not too hard to write a WebSocket or HTTP server in C, so if you are familiar with web programming, writing one of those might be a fun project.